1. Field of the Invention
The present invention relates to a progressive power lens wherein the dioptric power for distance vision is negative and a progressive surface is formed on the back surface thereof.
2. Description of the Related Art
Progressive power lenses which are utilized as spectacle lenses are known as lenses having a distance portion positioned on upper part of the lens, a near portion positioned on lower part of the lens, and an intermediate portion positioned between the near and distance portions with a continuous change of dioptric power from the near portion to the distance portion.
In such progressive power lenses, although the surface power of the distance portion and the surface power of the near portion are naturally different from each other, in the past, the surface power at a certain point on the main meridian in the main meridional plane was identical to the surface power at that same point in a plane perpendicular to the main meridional plane, in view of facilitating ease of manufacture thereof. In recent progressive power lenses, due to advancement in production technology, the surface power at a certain point in the plane perpendicular to the main meridional plane can be easily made different from the surface power at the certain point in the main meridional plane. Consequently, there have been various proposals made for the surface power for such progressive power lenses.
It is desirable, mainly from a cosmetic point of view, for progressive power spectacle lenses, and also any other types of spectacle lenses, to be formed (manufactured) as thin as possible. A spectacle lens can be thinned without changing the power thereof by employing a gentle base curve. However, if a gentle base curve is adopted for the purpose of thinning a progressive power lens, it is difficult to correct aberrations, particularly astigmatism. Due to this reason, an appropriate base curve is selected taking into account the correction for aberrations. However, due to such a conventional approach, the thickness of the progressive power lens cannot be made sufficiently small.